How individual Montagu's Harriers cope with Moreau's Paradox during the Sahelian winter
Résumé
1. Hundreds of millions of Afro-Palaearctic migrants winter in the Sahel, a semi-arid belt
south of the Sahara desert, where they experience deteriorating ecological conditions during
their overwintering stay and have to prepare for spring migration when conditions are worst.
This well-known phenomenon was first described by R.E. Moreau and is known ever since as
Moreau’s Paradox. However, empirical evidence of the deteriorating seasonal ecological conditions
is limited and little is known on how birds respond.
2. Montagu’s Harriers Circus pygargus spend 6 months of the year in their wintering areas in
the Sahel. Within the wintering season, birds move gradually to the south, visiting several distinct
sites to which they are site-faithful in consecutive years. At the last wintering site, birds
find themselves at the southern edge of the Sahelian zone and have no other options than facing
deteriorating conditions.
3. We tracked 36 Montagu’s Harriers with GPS trackers to study their habitat use and behaviour
during winter and collected data on the abundance of their main prey, grasshoppers, in Senegal.
Since grasshopper abundance was positively related to vegetation greenness (measured as normalized
difference vegetation index, NDVI), we used NDVI values as a proxy for prey abundance in
areas where no field data were collected. Prey abundance (grasshopper counts and vegetation
greenness) at wintering sites of Montagu’s Harriers decreased during the wintering period.
4. Montagu’s Harriers responded to decreasing food availability by increasing their flight
time during the second half of the winter. Individuals increased flight time more in areas with
stronger declines in NDVI values, suggesting that lower food abundance required more
intense foraging to achieve energy requirements. The apparent consequence was that Montagu’s
Harriers departed later in spring when their final wintering site had lower NDVI values
and presumably lower food abundance and consequently arrived later at their breeding site.
5. Our results confirmed the suggestions Moreau made 40 years ago: the late wintering period
might be a bottleneck during the annual cycle with possible carry-over effects to the breeding
season. Ongoing climate change with less rainfall in the Sahel region paired with increased
human pressure on natural and agricultural habitats resulting in degradation and desertification
is likely to make this period more demanding, which may negatively impact populations
of migratory birds using the Sahel.